Process for production of high-purity bis-β-hydroxyethyl terephthalate

ABSTRACT

To provide a process for producing a high-purity bis-β-hydroxyethyl terephthalate from crude terephthalic acid. 
     A process for producing high-purity bis-β-hydroxyethyl terephthalate by carrying out the following steps (1) to (4) in the mentioned order: (1) the step of esterifying crude terephthalic acid with ethylene glycol and/or ethylene oxide to form a reaction product containing bis-β-hydroxyethyl terephthalate, (2) the step of preparing a solution containing bis-β-hydroxyethyl terephthalate as the main solute and ethylene glycol as the main solvent by mixing the above reaction product obtained in the above step (1) with ethylene glycol as required and deionizing the solution to form a deionized solution of bis-β-hydroxyethyl terephthalate, (3) the step of distilling off substances having a boiling point lower than that of bis-β-hydroxyethyl terephthalate from the above deionized solution obtained in the step (2) to form crude bis-β-hydroxyethyl terephthalate, and (4) the step of subjecting the crude bis-β-hydroxyethyl terephthalate obtained in the step (3) to molecular distillation to distill out bis-β-hydroxyethyl terephthalate.

FIELD OF THE INVENTION

The present invention relates to a process for producing high-puritybis-β-hydroxyethyl terephthalate from crude terephthalic acid.

DESCRIPTION OF THE PRIOR ART

Bis-β-hydroxyethyl terephthalate is widely used as a raw material forpolyethylene terephthalate which is an extremely useful polyester in thefield of various moldings such as fibers, films and resins.

Polyesters, particularly polyesters comprising polyethyleneterephthalate as the main constituent are widely used for variouspurposes as described above and mainly produced by obtaining anintermediate containing bis-β-hydroxyethyl terephthalate through directesterification between terephthalic acid and ethylene glycol or throughan ester exchange reaction between a lower alkyl ester of terephthalicacid, especially dimethyl terephthalate, and ethylene glycol and thengenerally polycondensing the intermediate at a high temperature under ahigh vacuum. These production processes are currently put to practicaluse. Due to the recent diversified application of the polyesters basedon their excellent properties, requirements for obtaining higher qualityare being more and more diversified and advanced. One of thecharacteristic features of the polyesters is that they can be decomposedinto their raw materials which can be polymerized again to obtain newpolyesters. Therefore, it can be said that they are excellent in termsof resource saving.

Terephthalic acid as a raw material for terephthalate-based polyestersis generally supplied by oxidizing paraxylene. However, as terephthalicacid obtained by this method has low purity and is colored in mostcases, it cannot be used for the production of a polyester as it is.Therefore, it must be purified before it is used for the production of apolyester. However, it is not easy to purify terephthalic acid becauseof its low solubility in solvents.

That is, terephthalic acid is obtained by air oxidizing paraxylene atabout 175 to 230° C. in an acetic acid solvent using a catalystcomprising cobalt, manganese and bromine compounds in accordance with aso-called Amoco process. In this case, paratoluylic acid and4-carboxybenzaldehyde by-produced in the above oxidation step, metalssuch as cobalt and manganese used as a catalyst and halogen such asbromine remain in the obtained terephthalic acid, and the existence of acompound such as 3,6-dicarboxyfluorenone as a coloring impurity is alsoknown. Kagaku Binran, Applied Chemistry I, Process, pp. 535 (fifthedition published on Apr. 20, 1992) teaches that high-purityterephthalic acid is obtained by passing a water slurry solutioncontaining 10% or more of the above crude terephthalic acid through areactor maintained at a temperature of 250° C. or more to purify thecrude terephthalic acid by hydrogenation in the above Amoco process. Thecatalyst used in this case is a precious metal carried on active carbon.It is disclosed that the contents of 4-carboxybenzaldehyde and thecoloring component in the formed product thereby become very small.However, as terephthalic acid is generally obtained as a fine particlesolid, it is very difficult to handle it. Therefore, it is oftensupplied as a slurry containing ethylene glycol to an industrial-scaleproduction process. As easily anticipated by people having ordinaryskill in the art, it is difficult to remove impurities due to bigchanges in the properties and reactivity of the slurry caused bynon-uniformity in particle diameter.

A process in which terephthalic acid is purified as dimethylterephthalate which is a functional derivative relatively easy to bepurified and a polyester is obtained through an ester exchange reactionbetween it and ethylene glycol has also been employed. However, in thisso-called ester exchange process, it is necessary to recycle methanolwhich is by-produced inevitably, thereby imposing great restrictions onequipment and handling. In this case, there arises another problem thatan ester exchange catalyst is contained in the polyester product.Further, when a polyester obtained by the ester exchange process ismolded into a bottle, for example, the bottle is unsatisfactory in termsof color, transparency and moldability.

Such an attempt is also proposed to obtain bis-β-hydroxyethylterephthalate from terephthalic acid and purify this bis-β-hydroxyethylterephthalate by distillation.

JP-A 48-15846 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) discloses a process forobtaining high-purity bis-β-hydroxyethyl terephthalate by reactinghigh-purity terephthalic acid with ethylene oxide in the presence of abasic catalyst to obtain bis-β-hydroxyethyl terephthalate and distillingthis bis-β-hydroxyethyl terephthalate at a temperature of 200 to 350° C.under reduced pressure quickly and teaches that when quick distillationis carried out in this process in the presence of a phosphorus, sulfuror boron compound, the polymerization rate of bis-β-hydroxyethylterephthalate can be controlled and bis-β-hydroxyethyl terephthalate canbe obtained at a high yield. However, the results of studies conductedby the present inventor revealed that the purification ofbis-β-hydroxyethyl terephthalate containing various impurity ions cannotbe realized because these metal compounds separate out on theevaporation surface, thereby greatly preventing heat transmission withthe result that long-term continuous stable operation cannot becontinued substantially.

JP-A 49-36646 discloses a process for purifying bis-β-hydroxyethylterephthalate by distillation at a temperature of 140 to 190° C. bymaintaining the pressure of an output port at a range of 0.01 to 0.1mmHg. However, the results of studies conducted by the present inventorrevealed that the purification of bis-β-hydroxyethyl terephthalatecontaining various impurity ions cannot be realized even when thisprocess is carried, because polycondensation takes place with thepassage of operation time, thereby making impossible to carry outdistillation operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forproducing high-purity bis-β-hydroxyethyl terephthalate from crudeterephthalic acid.

It is another object of the present invention to provide a process forproducing high-purity bis-β-hydroxyethyl terephthalate which makes iteasy to remove impurities and possible to purity crude terephthalic acidby removing impurities contained in the crude terephthalic acid afterterephthalic acid is converted into bis-β-hydroxyethyl terephthalatewithout purifying the crude terephthalic acid.

Other objects and advantages of the present invention will becomeobvious from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a process forproducing high-purity bis-β-hydroxyethyl terephthalate by carrying outthe following steps (1) to (4) in the mentioned order:

(1) the step of esterifying crude terephthalic acid with ethylene glycoland/or ethylene oxide to form a reaction product containingbis-β-hydroxyethyl terephthalate;

(2) the step of preparing a solution containing bis-β-hydroxyethylterephthalate as the main solute and ethylene glycol as the main solventby mixing the above reaction product obtained in the above step (1) withethylene glycol as required and deionizing the solution to form adeionized solution of bis-β-hydroxyethyl terephthalate;

(3) the step of distilling off substances having a boiling point lowerthan that of bis-β-hydroxyethyl terephthalate from the above deionizedsolution obtained in the step (2) to form crude bis-β-hydroxyethylterephthalate; and

(4) the step of subjecting the crude bis-β-hydroxyethyl terephthalateobtained in the step (3) to molecular distillation to distill outbis-β-hydroxyethyl terephthalate.

The present inventor has paid attention to bis-β-hydroxyethylterephthalate obtained by the esterification of terephthalic acid andstudied a process for purifying this compound. As a result, he hassucceeded in reducing the total amount of ions contained in thisbis-β-hydroxyethyl terephthalate to an extremely low level by a specificmethod and purifying it by distillation. The present inventor has foundfor the first time that it is a very important technical factor toreduce the total amount of ions contained in crude bisβ-hydroxyethylterephthalate to be subjected to molecular distillation to an extremelylow level, and that it is a reason which purifying of bis-β-hydroxyethylcannot be attained by conventionally known processes.

The present inventor has conducted intensive studies to overcome theabove problem and has accomplished the present invention in the end.

The process of the present invention basically consists of four steps(1), (2), (3) and (4) as described above.

Crude terephthalic acid may be recovered terephthalic acid obtained bythe liquid-phase air oxidation or nitric acid oxidation of paraxylene,recovered terephthalic acid obtained by the depolymerization of apolyester, for example, the depolymerization of polyethyleneterephthalate and then acid neutralization, or terephthalic acidobtained by the hydrolysis of terephthalates. Or it may be a mixture oftwo or more out of these.

In the step (1), crude terephthalic acid is esterified with ethyleneglycol and/or ethylene oxide to give a reaction product containingbis-β-hydroxyethyl terephthalate.

The esterification reaction of crude terephthalic acid with ethyleneglycol itself is well known. This can be carried out by heating 1 mol ofcrude terephthalic acid and 1.1 to 2.0 mol of ethylene glycol at 220 to265° C. in the presence of an esterification catalyst. The reaction canbe carried out under normal pressure or increased pressure, for example,0.1 to 0.3 MPa. The reaction time is 2.5 to 4.0 hours. Theesterification catalyst is a Zn, Ca or Mg carboxylate, sodium alcoholateor the like.

The esterification reaction of crude terephthalic acid with ethyleneoxide itself is also well known. This can be carried out by heating 1part by weight of crude terephthalic acid and 0.5 to 0.7 part by weightof ethylene oxide at 170 to 190° C. in the presence of benzene and anesterification catalyst. The reaction is generally carried out underincreased pressure, for example, 1.0 to 1.2 MPa. The reaction time is1.5 to 2.5 hours. The esterification catalyst is triethylamine, forexample.

In the present invention, the deionization step (2) which will bedescribed hereinafter is carried out after the step (1). Before the step(2), the reaction product obtained in the step (1) can be depolymerizedwith ethylene glycol. In this case, the reaction product formed by thisdepolymerization is used as the reaction product used in the step (2).

In the present invention, the reaction product obtained in the step (1)is deionized to prepare a deionized solution of bis-β-hydroxyethylterephthalate in the step (2). Before this deionization, the reactionproduct obtained in the step (1) may be mixed with ethylene glycol asrequired to prepare a solution containing bis-β-hydroxyethylterephthalate as the main solute and ethylene glycol as the mainsolvent. When the concentration of the reaction product obtained in thestep (1) is higher than a level suitable for deionization which ispreferably 5 to 30 wt % of bis-β-hydroxyethyl terephthalate, it ispreferred to control the concentration of the reaction product to anappropriate range by mixing ethylene glycol.

In the step (2), the solution is contacted to a cation exchanger and/oran anion exchanger. The cation exchanger or anion exchanger may beparticulate, chain-like, fibrous or amorphous. For example, the solutioncan be contacted to a particulate ion exchanger by charging the ionexchanger into a column and let passing a solution composition throughthe column. The cation exchanger is preferably a cation exchange resinand the anion exchanger is preferably an anion exchange resin. Preferredexamples of the cation exchange resin include what have —SO₃H, —COOH or—N(CH₂COOH)₂ as a cation exchange functional group. They may be usedalone or combination. Commercially available cation exchange resinproducts include the SK series, PK series and WK series of Diaion (ofMitsubish Chemical Co., Ltd.), and the IR series and IRC series ofAmberlite (of Rohm and Haas Japan Co., Ltd.). Since the ion exchangefunctional groups of these commercially available products arestabilize& as a salt such as a sodium salt, they are generally convertedinto the above free acid groups when in use. Examples of the anionexchange resin include what have groups represented by the followingformulae as an anion exchange functional group.

They may be used alone or in combination. Commercially available anionexchange resin products include the SA series, PA series and WA seriesof Diaion (of Mitsubishi Chemical Co., Ltd.), and the IRA series andIRAC900 series of Amberlite (of Rohm and Harse Japan Co., Ltd.). Sincethe ion exchange functional groups of these commercially availableproducts are stabilized as what has a halogen anion and not a hydroxylion (OH⁻), they are generally converted into what have the abovehydroxyl group anion when in use.

Gel type anion exchange resins are divided into a cracked type and anon-cracked type. The non-cracked type is preferred because the amountof adsorbed bis-β-hydroxyethyl terephthalate is small.

Further, porous, so-called microporous, ion exchange resins which aresuperior in physical durability and exchange adsorption speed to the geltype ion exchange resins may also be used.

Either one or both of the cation exchanger and the anion exchanger maybe used. For example, when a solution composition containing a cationand an anion as impurities contains one of the ions in a much largeramount than the other ion and the content of the other ion is negligiblysmall, only an ion exchanger for removing the former ion may be used.

Generally speaking, both the cation exchanger and the anion exchangerare preferably used. In this case, the solution can be contacted to thecation exchanger and the anion exchanger simultaneously or sequentially.For example, simultaneous contact can be carried out by contacting thesolution to a mixture of the cation exchange resin and the anionexchange resin and sequential contact can be carried out by contactingthe solution to a column filled with the cation exchange resin and thena column filled with the anion exchange resin. Preferably, the solutionis first contacted to the cation exchanger and then the anion exchangersequentially.

The contact of the solution to the cation exchanger and the anionexchanger must be carried out at a temperature lower than the maximumuse temperature of the ion exchange resins, preferably 20 to 120° C.,more preferably 30 to 100° C., without the precipitation ofbis-β-hydroxyethyl terephthalate crystals from the solvent.

The amount of bis-β-hydroxyethyl terephthalate as a solute is preferably5 to 30 wt %, more preferably 8 to 25 wt %, particularly preferably 10to 20 wt % to carry out a deionization step on a stable solution.Ethylene glycol used in the esterification step is preferably used as asolvent. The amount of ethylene glycol forming a solvent is preferably70 to 95 wt %, more preferably 75 to 92 wt %, particularly preferably 80to 90 wt %. The types and amounts of an anion and a cation contained inthe solution change according to used substances such as crudeterephthalic acid and do not limit the present invention. The anion isgenerally contained in an amount of 20 to 3,000 ppm and the cation isgenerally contained in an amount of 2,000 to 3,000 ppm in most cases.

Contact may be carried out under normal pressure, reduced pressure orincreased pressure. It is needless to say that contact is carried outunder conditions such as concentration, temperature and pressure formaintaining the solution in a solution state.

In the present invention, before the step (2) after the step (1), duringthe step (2) or before the step (3) after the step (2), an ethyleneglycol solution containing bis-β-hydroxyethyl terephthalate ispreferably decolorized one time or plural times. In this decolorizationstep, it is preferred that the solution may be contacted to a decolorantsuch as conventionally known active carbon and then solid-liquidseparation may be made on the solution for decolorization.Decolorization is preferably carried out before ion exchanging becausethe mixing of ions from the decolorization step is prevented.

After the solution is contacted to the ion exchangers in the step (2), adeionized solution containing bis-β-hydroxyethyl terephthalate having asmall ion content, that is, a total content of an anion and/or a cationas impurities of 50 ppm or less, preferably 40 ppm or less, based onbis-β-hydroxyethyl terephthalate is obtained.

In the step (3), substances having a boiling point lower than that ofbis-β-hydroxyethyl terephthalate is distilled off from the thus obtaineddeionized solution containing bis-β-hydroxyethyl terephthalate. Thetemperature for distilling off the substances having a boiling pointlower than that of bis-β-hydroxyethyl terephthalate is, for example,170° C. or less, preferably 100 to 150° C. The pressure is preferably40,000 Pa (300 mmHg) or less, more preferably 20,000 Pa (150 mmHg) orless, particularly preferably 20 Pa (0.15 mmHg) to 130 Pa (1 mmHg) as anabsolute pressure.

This distillation is carried out until the content of ethylene glycol inthe system becomes preferably 10 wt % or less, more preferably 5 wt % orless, particularly preferably 2 wt % or less. By distilling off ethyleneglycol and diethylene glycol to that range, the substances having lowerboiling point than that of bis-β-hydroxyethyl terephthalate arecompletely removed and the distillation residue (crudebis-β-hydroxyethyl terephthalate) which is concentrated to such anextent that the molecular distillation of the next step (4) can beadvantageously carried out is obtained. There is such an advantage thatat least some of other impurities which may be existent in small amountsare removed while the substances having a boiling point lower than thatof bis-β-hydroxyethyl terephthalate are distilled off. The impuritiesinclude a third component such as cyclohexane dimethanol or diethyleneglycol when crude terephthalic acid is derived from a polyester,particularly a copolyester.

In the present invention, crude bis-β-hydroxyethyl terephthalateobtained in the step (3) is subjected to molecular distillation in thestep (4). The term “molecular distillation” as used herein means notequilibrium distillation or evaporation at a distillation temperatureand pressure but non-equilibrium evaporation by which evaporatedmolecules do not return to the evaporation surface substantially butmove to the condensation surface unilaterally. As for conditions forthis molecular distillation step, the temperature is preferably in therange of 130 to 250° C., more preferably 160 to 220° C. and the pressureis preferably 300 Pa (2.25 mmHg) (absolute pressure) or less, morepreferably 70 Pa (0.5 mmHg) (absolute pressure) or less. Further, theaverage residence time in this molecular distillation step is preferablyabout 2 hours or less, more preferably about 1.5 hours or less.

In the process of the present invention, bis-β-hydroxyethylterephthalate is preferably recrystallized one time or plural timesbetween the steps (2) and (3), between the steps (3) and (4), or afterthe step (4).

This recrystallization step may be carried out simultaneous withdecolorization as required. The solvent suitable for use in therecrystallization step is ethylene glycol, for example. Stated further,ethylene glycol is preferably used as the solvent in all the steps.

Bis-β-hydroxyethyl terephthalate obtained herein has extremely highpurity and a polyester obtained from this as a raw material can be usedsufficiently for various purposes.

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

EXAMPLES Example 1

83 kg (0.5 kgmol) of a substance obtained by washing with water anddrying a yellow tinted substance which contained crude terephthalic acidhaving 98.4% purity obtained by subjecting paraxylene to liquid-phaseair oxidation, 1.2% of 4-carboxybenzaldehyde, 0.2% of paratoluylic acid,0.1% of benzoic acid and 0.1% of ash and others to remove free aceticacid and 37.2 kg (0.6 kgmol) of ethylene glycol for industrial use werecharged into a 150-liter reactor, an esterification reaction was carriedout at 260° C. and 0.15×10³ KPa for 4 hours, and by-produced water wasdistilled off.

Thereafter, 510 kg of ethylene glycol for industrial use and 0.5 kg ofsodium methylate were added to depolymerize the above esterified productusing refluxing condition at a temperature lower than the boiling pointof ethylene glycol and normal pressure for 2 hours and maintained at70±3° C. for 3 hours to crystallize an unreacted oligomer and separateit by filtration. The oligomer was decolorized by the adsorption ofactive carbon at 80±3° C. and then deionized with an anion-cationexchange mixed bed to obtain a treated solution having a total cationweight of 9.4 ppm and a total anion weight of 0 ppm after deionization.(2 parts of cation exchange resin (Amberlite® IR120BH(HG)) and 1 part ofanion exchange resin (Amberlite® IRA96SB) are mixed and used with totalamount of 320L, column=550Φ×1,500 mm, rate of the solution passingthrough the column=120 L/hr, space velocity (SV)=0.375 hr⁻¹).

The obtained solution (having still light yellow color) was transferredto a jacketed and stirrer equipped vessel for mixing and was cooled to10° C. by circulating a brine in the jacket to precipitatebis-β-hydroxyethyl terephthalate crystals which were then obtained by afilter-press. The obtained crystals were heated at 85 to 90° C. to bemolten and placed in a thin film evaporator to distill off thelow-boiling substances together with the remaining ethylene glycol at13.33 Pa and 150° C., and then bis-β-hydroxyethyl terephthalate wassubjected to molecular distillation at a vacuum degree of 1.333 to 2.666Pa and 200° C. to obtain 121.6 kg of high-purity bis-β-hydroxyethylterephthalate. Other molecular distillation conditions and results-areshown in Table 1.

TABLE 1 1. amount of solution treated with molecular 123.7 distiller(kg) 2. treatment time of molecular distiller 68.3 (minutes) 3. amountof recovered purified bis-β- 121.6 hydroxyethyl terephthalate (kg) 4.recovery of purified bis-β-hydroxyethyl 98.3 terephthalate (%) 5. amountof formed oligomer (kg) 1.66 6. formation rate of oligomer (%) 1.34

The analytical results of the quality of the obtained bis-β-hydroxyethylterephthalate are shown in Table 2.

TABLE 2 1. optical density   <0.01 2. acid value (KOH mg/g)  0.38 3.saponification value (KOH mg/g) 440 4. melting point (° C.) 111.8 5.whiteness L = 98.9, a = −0.7, b = 1.1 6. total cation weight (ppm)  0.817. total anion weight (ppm)  0 8. bis-β-hydroxyethyl terephthalate (wt%)  97.99 9. mono-β-hydroxyethyl terephthalate (wt %)  1.27 10. oligomer(wt %)  0.74

The optical density in the above table is an index for the evaluation ofthe quality of bis-β-hydroxyethyl terephthalate and proportional to thecontent of a 10 colored product. It was obtained by measuring theabsorbance of a 10% methanol solution at a wavelength of 380 μm and acell length of 10 mm. The whiteness was measured with a color differencecolorimeter and represented by L (brightness), a (redness) and b 15(yellowness) of the Hunter method.

The low-boiling substances distilled off by the above thin filmevaporator contained diethylene glycol and a slight amount ofbis-β-hydroxyethyl terephthalate in addition to ethylene glycol andesters.

An oligomer having a low degree of polymerization, monohydroxyethylterephthalate and ash were observed in the residue in the evaporatorafter high-purity bis-β-hydroxyethyl terephthalate was distilled out.

What is claimed is:
 1. A process for producing high-puritybis-β-hydroxyethyl terephthalate by carrying out the following steps (1)to (4) in the mentioned order: (1) the step of esterifying crudeterephthalic acid with ethylene glycol and/or ethylene oxide to form areaction product containing bis-β-hydroxyethyl terephthalate; (2) thestep of preparing a solution containing bis-β-hydroxyethyl terephthalateas the main solute and ethylene glycol as the main solvent by mixing theabove reaction product obtained in the above step (1) with ethyleneglycol as required and deionizing the solution to form a deionizedsolution of bis-β-hydroxyethyl terephthalate; (3) the step of distillingoff substances having a boiling point lower than that ofbis-β-hydroxyethyl terephthalate from the above deionized solutionobtained in the step (2) to form crude bis-β-hydroxyethyl terephthalate;and (4) the step of subjecting the crude bis-β-hydroxyethylterephthalate obtained in the step (3) to molecular distillation todistill out bis-β-hydroxyethyl terephthalate.
 2. The process of claim 1,wherein the step of depolymerizing the reaction product obtained in thestep (1) with ethylene glycol is carried out between the above steps (1)and (2) and the reaction product formed in this depolymerization step isused as the reaction product in the step (2).
 3. The process of claim 1,wherein an ethylene glycol solution containing bis-β-hydroxyethylterephthalate is decolorized one time or plural times before the step(2) after the step (1), during the step (2), or before the step (3)after the step (2).
 4. The process of claim 1, whereinbis-β-hydroxyethyl terephthalate is recrystallized one time or pluraltimes between the steps (2) and (3), between the steps (3) and (4), orafter the step (4).
 5. The process of claim 1, wherein crudeterephthalic acid in the step (1) is an oxidized product of paraxylene.6. The process of claim 1, wherein crude terephthalic acid in the step(1) is a depolymerized product of polyethylene terephthalate.
 7. Theprocess of claim 1, wherein crude terephthalic acid in the step (1) is aproduct obtained by the alkali decomposition of polyethyleneterephthalate and the subsequent acid neutralization.
 8. The process ofclaim 1, wherein crude terephthalic acid in the step (1) is a hydrolyzedproduct of a terephthalate.